I very highly recommend getting familiar with the 5 basic objects, the cube/box being the most important to master. Having said that you need to respect the perspective when you draw these. Also, before taclking the cube, you should master drawing fast fluid straight lines. That in itself requires specific ergonomic and posture considerations as well.
Since your lines are not there yet, let's start with mastering drawing straight lines.
3 keys to drawing straight lines:
1. Do not lock any joints on your arm. If you lock/rest your elbow, you will just end up drawing big arcs. If you lock/rest your palm, you will just draw smaller arcs. In order to draw long straight lines, you need to make sure you are not resting or locking your wrist, elbow nor shoulders. You let your hand glide and guide the line's trajectory.
2. You must draw the line fast. Slower you draw, more chance you give the line to go crooked. Speed is the key to straight lines.
3. Look at where you want the line to go, not where your hand is. Aim at a target, look there, and draw and fast line while not resting any part of your arm.
Practice doing parallel lines on a large (minimum 17x11" bond paper) from all the way left to all the way right. Fill the page. Make adjustments after each line to keep the lines straight. Keep it loose and fast.
MASTER THE LINE BEFORE ANYTHING ELSE.